The plan build article - 19/5/10
The Bats (Bramhall Area Thermal and Slope) club holds bi-weekly glider competitions for two-metre gliders, the rules of which specify a bungee launch to a 6-minute max with one point per second flown but with no penalty for over-flying. A bonus of 25 points is awarded for landing, all or part, in the 25m (82') diameter circle. That's easy enough, and the most popular model flown is the 'Gentle Lady' variety, which goes up the line nicely in light winds and hangs around in light lift. Ideal for our evening competitions, most of which are flown in little or no wind. Under such conditions a lightweight model is ideal, provided it's strong enough to stand a good blow without wing flutter and able to carry ballast. And anything lighter than the usual 765 - 1020g (27 - 36oz) should also provide an advantage. Anyway, this set me thinking, and I donned my designer's cap to come up with such a model.
V-tailed and based on a fishing rod fuselage, Toovix used foam, spruce-sparred wings with lightning holes cut through, and was reasonably successful. Unfortunately, however, weight was still a penalty. 2Vix2 was consequently devised, again with a fishing rod fuselage and V-tail but this time using a strong, rigid and lightweight wing that had a built-up geodetic construction employing one-piece carbon tube and carbon rod spars. Initial trials used rubber band mounting but this was soon converted to a single M4 screw, on the trailing edge. The section used was the SD 7037, which copes well with a wide variety of conditions but is a bit more difficult to build, having a thin trailing edge.
Initially the 2Vix2 (like the first Toovix) used two mid-size servos operating the V-tail through pushrods. However, I discovered that two 6g servos mounted in the tail with a four-wire cable down the fuselage offered a weight saving, coupled with the fact that more precise operation was evident at the control surfaces. The nose on the 2Vix2 is quite long, and a removable nose cone provides access to a four-cell 500AR in-line NiCad pack. A pleasant surprise was that this functional glider turned out slightly under 567g (20oz), and with most other 2M gliders coming to the line in excess of 907g (32oz), I was very satisfied. Mind you, I was still looking at other ideas to simplify and lighten the model, the result being the version that’s presented to you here. This latest incarnation of the 2Vix2 has a single tube fuselage and conventionally-built 'rib and spar' wing, as opposed to the geodetic arrangement of its predecessor.
The centre panel of this new wing is 1m (39.4") long and built in one piece in order that the carbon spars can run right across for maximum strength. Of course, if you wish, the fuselage could be modified so that the wing can be made in two halves and plugged into a 6swg joiner with incidence keepers, but that's up to you. I prefer a one-piece wing, so let's go build it.
The necessary carbon bits are readily available from your local kite shop, though I actually sourced my carbon from a 'Decathlon' sports store. If you have problems getting hold of the necessary then e-mail me and I'll do my best to help. Just about any sort of thread will do to spirally wind the spar. The section is again the SD 7037, and pre-construction tasks include cutting a set of wing rib templates and sanding a t.e. from 1" x 3/16" medium hard balsa. Splice two lengths together on centre to make up the 1m centre-section. You'll also need two more pieces for the tips, each 500mm (20") long.
The 3/16" l.e. and 1" wide t.e. are pinned to the board as shown, but note that the front of the t.e. is packed up by bits of 1mm ply to follow the undercamber. Next glue in the ribs, notching these 1/16" into the t.e.; note that the outer ribs have dihedral, to meet the outer panels. When set, notch out the upper surface of the ribs for the top carbon rod and glue this in with cyano'. When this has cured, turn the panel over so that you can notch out the underside for the other 3mm rod. Cyano’ this in place, then fill in the gaps between the carbon rods and ribs with medium / hard balsa. Spirally wind with thread as shown, and wet-out with thin cyano’ to seal. The upper surface of the l.e. has to be sanded such that the l.e. sheeting follows the section profile as shown. Pin it down (remembering the packing pieces), glue up, and leave it overnight.
The next task is to add the lower sheeting, which butts against the l.e. Sheet the centre-section after adding hardwood blocks to reinforce the area where the mounting screw goes through. Note the 1/4 x 1/8" top and bottom balsa spars that support the covering between the main spar and the t.e.; not load bearing but a useful addition anyway. The balsa tips are straightforward enough, reinforced with ply to strengthen the l.e. at the tip's change of taper. With the tips in place and sanded to shape, and when you've completed the three panels, it's time to glue them together.
Okay, when the glue's dried and you're happy with the construction and finish, you can apply your favourite covering and colour. Initially I used blue and green translucent film, not because I particularly liked the colour but simply because I had it in stock!
Consult the plan, cut 3/16" balsa sheet into strips and away you go - quick and easy. The clever bit comes with the carbon rod offset in the root; when holes are marked out and drilled in the rear of the fuselage tube, these are glued in top and bottom and also at the crossover, making a sound mounting. The tailplane halves are glued in at an included angle of 120°, so take care when marking and drilling. Better still, use a card jig. When set, the carbon pegs can be cyano' glued and trimmed back to the tube.
Mark and cut out for the micro servos and reinforce the edges of the cuts using thin cyano’. Add lightweight horns to the control surfaces as shown. I used a JR 700 Rx, which drops neatly into the fuselage. Whichever receiver you use, it can be packed forward or backwards for fine trimming of the C of G. In the absence of sufficient servo lead length I used four strands stripped from a six-core burglar alarm cable. Simply run two wires for servo signal, and gang positive and negative together for the required length inside the boom.
For setting up the servos you'll need to be able to switch on the radio with the right mixers in operation. You'll then be able to neutralise the system, carefully Z-bend the pushrods and locate the servos to suit, meaning that adjusters aren't necessary. Easy!
This begins life as a section of fishing rod blank, about 3/4" inside diameter at the big end and 38" long. This has to be big enough to house the NiCad pack, for which, once again, I used four 500ARs soldered in-line. Cut out the front upper (as shown) to accept the Rx and also allow the NiCad lead to pass through. The fuselage sides are from 1.5mm (1/16") ply, glued in flush to the centreline of the tube. Cut and glue spacers to suit, noting that a hardwood or ply block is also to be glued in, with two holes drilled to accept the wing mounting pegs.
The upper length under the wing is shaped to fit the section undercamber, and a 1.5 x 51mm (1/16" x 2") wide top deck is added, including an M4 T-nut and support block where shown.
There's a removable hatch required for access to the Rx and NiCad lead, and here you can use either balsa block or snap-in ply to choice.
Before gluing everything together do check that it's all square and aligned properly. Simply eyeballing the assembly might do for some folk but it's best to do it right first time, so jig it up and be sure. For the tow hook I modified a square cup hook, screwed into a piece of ply let into the bottom of the fuselage tube.
With the wing already film covered, do the same for the V-tail before whipping out the filler and spray paint for the fuselage. Rig the model and check the C of G; you may need some lead, but it won't be much. Add a switch harness for the R/C, run the receiver aerial along the outside of the boom, and you're done! Set up the V-tail for all the rudder that you can get and, for starters, mix in about ±12mm (1/2") of elevator travel.
A hand-launch to verify the glide is all that's needed before putting Vix2 up the line; lots of tension is fine, as you're unlikely to break that wing. With the nose well up it will probably climb the first 100' above your head, and a gentle weave will help keep tension on for a higher release.
I'll discuss 'ping' launches at a later date, but the three-position elevator trim switch system that I use is worth a mention here. I can't obviously deal with every Tx mix, but I set the elevator in the neutral position for normal glide, 'up' trim is applied from halfway up the line to increase the tension, and forward trim for 'down' gives increased penetration for coming back from downwind. Around ±1.5mm (1/16") from neutral is slightly too much, but experiment and change to suit your model and conditions.
2Vix2 is quick to build, light, strong, and a good performer. If the one-piece wing is a problem for storage or transportation then simply build in two extra ribs and a spacer with a couple of joiner tubes, and saw through on the centreline when you've done. You’ll also need to alter the fuselage sides accordingly. I'd like to know how you get on with your 2Vix2 for who knows, maybe this could be a springboard of the return to the once popular 2M postal competitions, which were always enjoyable.
Aircraft type: Thermal soarer
Designed by: Chas Gardiner
Wingspan: 2m (79'')
Fuselage length: 1060mm (413/4'')
Wing area: 0.4sq. m. (4.1sq. ft.)
All-up weight: 595g (21oz)
Wing loading: 1.6kg / sq. m (5.1oz / sq. ft.)
Rec’d radio: Tx with mixer
Control functions: V-tail (rudder, elevator)
Control deflections: V-tail mix, elevator ±12mm (1/2''), rudder: as much as possible!
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